Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under a person's clothing. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some kinds of explosives, however, a wide range of dangerous objects exist that cannot be detected with these devices. Plastic and ceramic weapons developed by modern technology increase the types of non-metallic objects that security personnel are required to detect. The alternative of manual searching of subjects is slow, inconvenient, and would not be well tolerated by the general public, especially as a standard procedure in, for example, airports.
Radiation exposure is an important consideration in X-ray concealed object detection systems. The United States National Council on Radiation Protection (NCRP), in NCRP Report No. 91, "Recommendations on Limits for Exposure to Ionizing Radiation", 1987, addresses this issue. In this report, the NCRP states that a radiation exposure of less than 1000 microRem per year in excess of environmental levels is negligible, and efforts are not warranted at reducing the level further. Persons employed in high security or secured facilities, or those who frequently travel by airlines, may be subjected to many hundred security examinations per year. A yearly radiation exposure limit of 1000 microRem permits a single scan exposure within the range of 1 to 10 microRem for the general public. In accordance with the NCRP recommendations, radiation levels significantly higher than this present a non-trivial health risk.
Known prior art X-ray systems suggested for detecting objects concealed on persons have limitations in their design and method which prohibit them from achieving the low dose and high image quality which are prerequisites to commercial acceptance. For example, radiant energy imaging systems that detect concealed objects carried on a person or in a container scan pencil beam of X-rays through the subject's body where the beam is transmitted or absorbed depending upon the concealed object, if any. A detector is scanned vertically behind the subject to collect the transmitted X-rays. The X-ray tube potential for these systems is set at up to 150 Kilovolts and is specifically chosen to transmit X-rays through the person being examined. This technique requires the subject to be exposed to a substantial radiation dosage, especially if the subject is scanned often, e.g., a frequent flyer.
Flying spot scanning systems for baggage inspection, also known in the art, utilize X-ray-induced fluorescence to permit detection of concealed objects. Since fluorescence is dependent upon atomic number, each object emits a fluorescent radiation line which is unique to its atomic number Z. Scattered, transmitted and fluorescence signals are generated to distinguish objects from the background. The energy level of the source must be sufficient to produce a large number of X-rays, at a selected fluorescent radiation line, that escape the object. Thus, it may be necessary to expose the object to relatively high X-ray energy in order to detect certain materials, which would be unacceptable for personnel inspection systems.
Inspection systems which operate at low levels of radiation exposure are limited in precision by the small number of X-rays that can be directed against persons being searched. X-ray absorption and scattering further reduces the number of X-rays available to form an image of the person and any concealed objects. In prior art systems, this low number of detected X-rays has resulted in unacceptably poor image quality. In addition, these systems do not adequately detect plastics, ceramics, explosives, illicit drugs, and other non-metallic objects. One reason in particular is that these materials share the property of a relatively low atomic number (low Z). Low Z materials present a special problem in personnel inspection because of the difficulty in distinguishing the low Z object from the background of the subject's body which also has low Z.
Other baggage inspection systems known in the art include detectors for both transmitted and backscattered X-rays to independently produce signals from the same incident beam. The separate signals may then be used to enhance each other to increase the system's accuracy in recognizing low Z materials. Clearly, with the incident beam being of sufficient energy to provide both transmitted and backscattered signals, the X-ray energy must be relatively high, making such a system undesirable for personnel inspection. U.S. Pat. No. 5,181,234, assigned to the assignee of the present invention, relates to an X-ray inspection system.
It would be desirable to provide a walk-through X-ray inspection system that automatically scans an individual from a variety of angles to provide a thorough search of the individual for metals as well as low Z materials, but which does not expose the subject to radiation doses significantly higher than normal environmental radiation levels. It is to such a system that the present invention is directed.